Mapped variable smoothing evolution method and device

ABSTRACT

A variable mapped on an orientation of a device is smoothed to have a gradual evolution by adjusting the estimated orientation of the device obtained via sensor fusion or other sensor processing to take into consideration a current measured angular velocity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No. 14/888,255, filed Oct. 30, 2018, which is a National Stage of International Application No. PCT/US14/36142, filed Apr. 30, 2014, which claims the benefit of U.S. Provisional Application No. 61/818,116, filed May 1, 2013; the contents of each of which are incorporated by reference herein.

TECHNICAL FIELD

The present inventions generally relate to methods and dedicated apparatuses outputting a variable mapped on a device orientation in a non-inertial reference system, with the device orientation being estimated using measurements of motion sensors (such as 3D accelerometers and gyroscopes) and a magnetometer or other similar sensors including cameras.

BACKGROUND

As described in WO 2012/044964, yaw, roll and pitch angles of a device in a gravitational reference system may be evaluated using measurements of a magnetometer and other motion sensors (accelerometers, gyroscopes) attached to the device. These methods include:

-   -   determining a measured 3D magnetic field, a roll, a pitch and a         raw estimate of yaw in the body reference system based on the         received measurements,     -   extracting a local 3D magnetic field from the measured 3D         magnetic field, and     -   calculating yaw angle of the body reference system in the         gravitational reference system based on the extracted local 3D         magnetic, the roll, the pitch and the raw estimate of yaw using         at least two different methods,         wherein estimated errors of the roll, the pitch, and the         extracted local 3D magnetic field affect an error of the yaw         differently for the different methods.

A rotation matrix corresponding to the yaw, roll and pitch angles may be expressed as a quaternion (conversion between a rotation matrix corresponding to rotations around three orthogonal axes and a quaternion is known). The result of the sensor fusion methods described in WO 2012/044964 may be expressed as a quaternion.

Motion sensors may include gyroscopes or other sensors configured to measure angular velocities. The quaternion result from sensor fusion method (referred to as “fusion quaternion” hereinafter) is the best estimation of rotation angles (yaw, roll and pitch) based on all available sensor data. The sensor orientation (i.e., the output quaternion) may be mapped on a variable such as a position of a cursor on a screen or an image displayed to a user of a gaming system. Therefore, the output quaternion should be as accurate, stable (e.g., varying smoothly rather than “jumpy”) and consistent with all sensor indications as achievable. However, at certain moments, the angle estimations (which may be expressed as the fusion quaternion) do not agree with direct angular velocity measurements and, therefore, are not suitable for direct and indiscriminate use.

When a conflict between the fusion quaternion and the measured angular velocity arises, simple approaches to overcome this conflict are (1) to use the fusion quaternion directly or (2) to use measured angular velocity only.

One problem with the first approach (using the fusion quaternion directly) is that the fusion quaternion is not always continuous due to many reasons, such as magnetic field interference, linear acceleration, accelerometer saturation, etc. Another problem is that a fusion quaternion may keep moving while the device is still, due to delays introduced in the fusion process. Moreover, the fusion quaternion may also move in a direction different from that indicated by the angular velocity, due to delay or adjustment.

The second approach (using only angular velocity) is also problematic. The integration drift over a long period of time may cause a large misalignment between the integrated angular position and the device's true orientation.

Accordingly, it would be desirable to provide apparatuses and methods that advantageously make use of the fusion quaternion while also maintaining compatibility with measured angular velocity and avoid the problems identified above relative to the first and second approaches.

SUMMARY

Method and apparatuses according to various embodiments perform fusion quaternion smoothing in view of a measured angular velocity, and output a smoothed version of the fusion quaternion. The smoothed quaternion always moves in a manner that largely agrees with the measured angular velocity, while also matching the fusion quaternion in the long run. This quaternion smoothing has the advantage that the smoothed quaternion is consistent with the angular velocity within an angle limit and a scale limit, so that any discrepancy or inconsistency is not noticed by users viewing a cursor or an image determined using the smoothed quaternion. Also, the smoothed quaternion follows the fusion quaternion in the long run, eliminating long-term misalignment.

According to an embodiment, there is a method for smoothing evolution of a variable depending on an orientation of a device. The method includes determining an adjusted angular velocity based on a measured angular velocity and an estimated angular velocity so as to satisfy one or more predefined constraints. The method further includes determining a current value of the variable according to an adjusted estimate of the device's current orientation, obtained using the adjusted angular velocity.

According to another embodiment, there is a method for smoothing evolution of a variable depending on an orientation of a device. The method includes determining an expected orientation using a previous orientation of the device and a measured angular velocity. The method further includes determining a current value of the variable according to an adjusted estimate of the device's current orientation, obtained so as to be as close as possible to an estimate of the device's current orientation, but with an angle between the adjusted estimate of the current orientation and the expected orientation to be less than a maximum angle.

According to another embodiment, there is a gaming system configured to display an image to a user according to an orientation of a device. The gaming system includes (i) sensors mounted on the device and configured to acquire information leading to a measured angular velocity and an estimate of a current orientation of the device, and (ii) a data processing unit. The data processing unit is configured to determine (A) an adjusted angular velocity based on the measured angular velocity and the estimated angular velocity so as to satisfy one or more predefined constraints, (B) an adjusted estimate of the device's current orientation, obtained using the adjusted angular velocity, and (C) the image to be displayed to the user according to the adjusted estimate of the current orientation.

According to yet another embodiment, there is an information system controlled by orientation of a device. The system includes sensors mounted on the device and configured to acquire information leading to a measured angular velocity and an estimate of a current orientation of the device, and a data processing unit. The data processing unit is configured to determine (A) an adjusted angular velocity based on the measured angular velocity and the estimated angular velocity so as to satisfy one or more predefined constraints, (B) an adjusted estimate of the device's current orientation, obtained using the adjusted angular velocity, and (C) a position of a cursor on a screen based on the device's current orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

FIG. 1 is an illustration of the problem solved by quaternion smoothing;

FIG. 2 is a flowchart of the quaternion smoothing method according to an embodiment;

FIG. 3 is a block diagram of an angular velocity alignment method according to an embodiment;

FIG. 4 is a graph of smoothed angular velocity versus measured angular velocity according to two embodiments;

FIG. 5 is a block diagram for determining an adjusted angular velocity according to an embodiment;

FIG. 6 illustrates determining the direction of the adjusted angular velocity according to an embodiment;

FIG. 7 illustrates tremor suppression according to an embodiment;

FIG. 8 is a flowchart of a method according to another embodiment; and

FIG. 9 is a block diagram of an apparatus according to an embodiment.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of systems using sensor fusion in which a magnetometer and motion sensors are used to evaluate orientation of a device.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

FIG. 1 illustrates, as a starting point, the tip of a previous smoothed quaternion q_(s,n-1) (here s stands for smoothed and n−1 indicates a previous moment). Sensor fusion method yields fusion quaternion q_(f,n) (here f stands for fusion and n indicates the current moment, after n−1), but the measured angular velocity ω_(n) (considering the time interval n−1 to n unitary here and hereinafter when referring to angular velocity) indicates instead an expected quaternion q_(ω,n) (here ω indicates that measured angular velocity was used to infer this quaternion and n for the current moment). Methods according to various embodiments described in this section determine a current smoothed quaternion q_(s,n) (here s stands for smoothed and again n indicates the current moment) located between quaternions q_(f,n) and q_(ω,n). In various applications, a variable x (not shown) is mapped on the current smoothed quaternion q_(s,n). For example, variable x may be a cursor position on the screen. The evolution of variable x is smoothed (i.e., sudden changes are implemented gradually) by using smoothed quaternion q_(s,n).

FIG. 2 is a flowchart illustrating a method 200 for smoothing evolution of a variable depending on an orientation of a device. Method 200 includes, at S210, determining an adjusted angular velocity (e.g., Ω_(s,n), in FIG. 1) based on a measured angular velocity (ω_(n)) and an estimated angular velocity (Ω_(f,n)), so that the adjusted angular velocity satisfies one or more predefined constraints. Method 200 further includes, at S220, determining a current value (x) of the variable according to an adjusted (smoothed) estimate of the current orientation (q_(s,n)) of the device obtained using adjusted angular velocity (Ω_(s,n)). Details of these steps according to various embodiments are described below.

FIG. 3 is a block diagram of a quaternion smoothing method 300 according to an embodiment. First, in block 310, estimated angular velocity Ω_(f,n) is determined based on the fusion quaternion q_(f,n) (obtained based on magnetometer and motion sensor measurements using one of the methods described in WO 2012/044964) and the previous smoothed quaternion q_(s,n-1). This estimated angular velocity Ω_(f,n) is, in general, different from the current angular velocity ω_(n) measured, for example, by a gyroscope.

In block 320, the adjusted angular velocity Ω_(s,n) leading (when integrated) to the smoothed quaternion q_(s,n) is determined as follows. The direction of adjusted angular velocity Ω_(s,n) is set to match that of the estimated angular velocity Ω_(f,n), if the angle between estimated angular velocity Ω_(f,n) and current (measured) angular velocity ω_(n) is less than a predetermined angle limit θ_(max). The direction of adjusted angular velocity Ω_(s,n) is set at θ_(max) from the current (measured) angular velocity ω_(n), if the angle between estimated angular velocity Ω_(f,n) and current (measured) angular velocity ω_(n) exceeds the predetermined angle limit θ_(max). The adjusted angular velocity Ω_(s,n) is in the same plane as estimated angular velocity Ω_(f,n) and current (measured) angular velocity ω_(n).

Further, the magnitude of estimated angular velocity Ω_(s,n) is set equal to the magnitude of Ω_(f,n)'s component along the angular velocity's direction, if the ratio between the magnitude of Ω_(f,n)'s component and the angular velocity ω_(n) is within a predetermined scale range, and the magnitude is set to a minimum/maximum value (depending upon which limit of the range is exceeded) otherwise.

Then, in block 330, adjusted angular velocity Ω_(s,n) is integrated from the previous smoothed quaternion q_(s,n-1) to obtain the current smoothed quaternion q_(s,n). Due to the manner in which it is obtained, smoothed quaternion q_(s,n) is always closer to the current fusion quaternion q_(f,n) than quaternion q_(ω,n-1) indicated by measured angular velocity ω_(n).

A few special situations require particular treatment. A first special situation is when the difference between angular velocity ω_(n) and estimated angular velocity Ω_(f,n) exceeds a predetermined threshold. In one embodiment, fusion quaternion q_(f,n) is output in the first special situation. However, other embodiments proceed differently in this first special situation.

A second special situation is when the magnitude of angular velocity ω_(n) is less than a minimum velocity ω_(min) (e.g., the user may intend to hold the device still). In one embodiment, the output quaternion q_(s,n) is made equal to the previous smoothed quaternion q_(s,n-1), i.e., the smoothed quaternion angular velocity Ω_(s,n) is zero. When angular velocity ω_(n) exceeds the pre-determined minimum ω_(min), the smoothed angular velocity Ω_(s,n) increases suddenly as illustrated by line 410 in FIG. 4. In one embodiment, this step increase may be smoothed to a more gradual increase as illustrated by curve 420. However, other embodiments may proceed differently in this second situation.

Returning now to FIG. 3, tremor suppression (alias smoothed quaternion stabilization) block 340 is optional. The purpose of tremor suppression is to output a stationary quaternion q′_(s,n) when the current smoothed quaternion q_(s,n) is very close to the previous smoothed quaternion q_(s,n-1) (i.e., the smoothed quaternion is approximately stationary). Such a situation (i.e., second special situation discussed above) occurs, for example, when a user intends to keep the device stationary, but small motions due to hand tremor are sensed. Stabilization is not necessary when the sensor readings are filtered to remove noise and user's tremor. However, in case of severe user's tremor or when sensor readings are not filtered, tremor suppression is beneficial especially when the output quaternion is used to determine a cursor position (i.e., the variable is the cursor's position).

An embodiment of block 330 for determining adjusted angular velocity Ω_(s,n) (e.g., step S210 in FIG. 2) is illustrated FIG. 5, and further explained in FIG. 6. Determining adjusted angular velocity Ω_(s,n) starts from measured angular velocity ω_(n) and estimated angular velocity Ω_(f,n) (i.e., these are the block's inputs). This processing encompasses two aspects: determining the magnitude and the orientation of adjusted angular velocity. These determinations are subject to two constraints: (A) the difference between the magnitude of the adjusted angular velocity and the measured angular velocity to be limited (i.e., the magnitude is within a predetermined range relative to measured angular velocity's magnitude), and (B) an angle between the estimated angular velocity Ω_(f,n) and the measured angular velocity ω_(n) to be limited.

Block 510 calculates the scalar product of estimated angular velocity Ω_(f,n) and measured angular velocity ω_(n). The scalar product may be used to determine the cosine of angle θ between estimated angular velocity and measured angular velocity:

$\begin{matrix} {{{\cos \mspace{14mu} \theta} = \frac{\omega_{n} \cdot \Omega_{f,n}}{\left| \omega_{n}||\Omega_{f,n} \right|}},} & (1) \end{matrix}$

where ∥ indicates magnitude indicates scalar product.

Block 520 then calculates an initial scale α according to the formula (2):

$\begin{matrix} {\alpha = {\frac{\left| \Omega_{f,n} \middle| {\cos \mspace{14mu} \theta} \right.}{\left| \omega_{n} \right|}.}} & (2) \end{matrix}$

The numerator of this ratio is the magnitude of the estimated angular velocity's projection along the measured angular velocity (i.e., from q_(s,n-1) to point P in FIG. 1). Projecting estimated angular velocity Ω_(f,n) is represented in FIG. 1 using a dashed line, which starts at the tip of estimated angular velocity Ω_(f,n), is perpendicular to the measured angular velocity ω_(n), and ends in point P. The initial scale α may then be adjusted in block 530 to a scale value α′ within the range (MinScaling, MaxScaling):

$\begin{matrix} {\alpha^{\prime} = \left\{ {\begin{matrix} {MaxScaling} & {{{If}\mspace{14mu} \alpha} > {MaxScaling}} \\ {MinScaling} & {{{If}\mspace{14mu} \alpha} < {MinScaling}} \\ \alpha & {{for}\mspace{14mu} {all}\mspace{14mu} {other}\mspace{14mu} \alpha} \end{matrix}.} \right.} & (3) \end{matrix}$

The magnitude of adjusted angular velocity Ω_(s,n) is then:

|Ω_(s,n)|=α′|ω_(n)|.  (4)

FIG. 1 illustrates the situation in which α=α′ and, therefore, the locus of the adjusted angular velocity's tip is the dashed portion of a circle with the center in q_(s,n-1) and passing through point P.

To determine the direction of the adjusted angular velocity, in block 540, a set of unit vectors is defined as illustrated in FIG. 6. Unit vector v₀ is along the measured angular velocity ω_(n). Unit vector v₁ is along the estimated angular velocity Ω_(f,n). In other words, measured angular velocity ω_(n) and estimated angular velocity Ω_(f,n) may be written as:

ω_(n) =v ₀|ω_(n)|  (5)

Ω_(f,n) =v ₁|Ω_(f,n)|.  (6)

Unit vector v₂ is the normalized component of vector v₁ on a direction perpendicular to vector v₀. Before normalization, vector v₁'s component on the direction perpendicular to vector v₀ is v₂ ^(u), and it is equal to the difference between the vector v₁ and vector v₀'s projection along v₁ (i.e., v₁ cos θ):

$\begin{matrix} {v_{2}^{u} = {v_{1} - {v_{0}\mspace{14mu} \cos \mspace{14mu} \theta}}} & (7) \\ {v_{2} = {\frac{v_{2}^{u}}{\left| v_{2}^{u} \right|}.}} & (8) \end{matrix}$

The direction of the adjusted angular velocity Ω_(s,n) is unit vector v defined using v₀, v₂ and a predefined parameter θ_(max) to be

v=v ₂ sin θ_(max) +v ₀ cos θ_(max).  (9)

Thus, unit vector v is coplanar to unit vectors v₀, v₁, at θ_(max) from unit vector v₀ toward unit vectors v₁. Other alternative methods may be used to obtain unit vector v.

In one embodiment, at block 550, if angle θ between estimated angular velocity Ω_(f,n) and measured angular velocity ω_(n) is less than θ_(max) then v=v₁. Otherwise (if angle θ exceeds θ_(max)) v is calculated with formula (9).

At block 560, outputs of blocks 530 and 550 (i.e., magnitude and orientation) are combined to obtain adjusted angular velocity Ω_(s,n) as:

Ω_(s,n)=|Ω_(s,n) |·v.  (10)

The above-discussed methods for determining adjusted angular velocity can also be used when smoothed angular position is not represented by quaternions, but by Euler angles or rotation matrices.

The amount of quaternion difference between smoothed quaternion q_(s,n) and fusion quaternion q_(f,n) is influenced by parameters θ_(max), MinScaling and MaxScaling. In some embodiments, these parameters may be fixed, but in other embodiments these parameters may be dynamically adjusted to obtain a smoother result. For example, the adjustment may be made larger when the difference between estimated angular velocity and measured angular velocity is large.

Under some circumstances (e.g., dramatic changes of the fusion quaternion q), the adjustment might be in one direction for a first moment, and change to the opposite direction for a second (next) moment. To reduce radical and frequent changes, in one embodiment, the variation of the quaternion adjustment from one moment to the next may be limited.

Returning now to FIG. 3, in one embodiment, optional tremor suppression block 340 operates based on a circular backlash algorithm. Although the quaternion is a 4D variable, the concept of circular backlash is illustrated in FIG. 7 as a 2D drawing. Circle 710 has as its center the previous stabilized quaternion q′_(s,n-1) and a radius m (i.e., a maximum angular velocity multiplied by an unitary time interval from n−1 to n). If the current adjusted quaternion q_(s,n) is inside circle 710, then the stabilized quaternion q′_(s,n) is made equal to the previous stabilized quaternion q′_(s,n-1) (i.e., q′_(s,n)=q′_(s,n-1)) so that the stabilized quaternion q′_(s,n) and the circle surrounding it remain the same. If the current adjusted quaternion q_(s,n) is outside circle 710, a new circle 720 is shifted relative to circle 710 with the least amount necessary to cover q_(s,n). Stabilized quaternion q′_(s,n) is the center of this new circle 720.

Thus, if q_(s,n)−q′_(s,n-1)<m (i.e., q_(s,n) is inside the circle), q′_(s,n)−q′_(s,n-1)=0, otherwise q′_(s,n)−q′_(s,n-1)=q_(s,n)−q′_(s,n-1)−m (i.e., the output quaternion q′_(s,n) and the circle are shifted with the least amount necessary to cover q_(s,n)).

The current smoothed quaternion q_(s,n) may be obtained in the quaternion domain directly, without using angular velocities (i.e., skipping blocks 310 and 330 in FIG. 3). FIG. 8 is a flowchart of a method 800 according to an embodiment, in which an expected orientation q_(ω,n) is determined using measured angular velocity ω_(n) and a previous orientation q_(s,n-1), at S810. Then, at S820, method 800 includes determining a current value x of the variable according to an adjusted estimate of the current orientation q_(s,n) of the device which is obtained so as to minimize the angle between it and the estimate of a current orientation q_(f,n) of the device (e.g., according to the sensor fusion), while satisfying the requirement that the angle φ between the adjusted estimate of current orientation q_(s,n) and expected orientation q_(ω,n) be less than a maximum angle φ_(max). For example, if the angular difference φ between q_(f,n) and q_(ω,n) is less than φ_(max), then q_(s,n)=q_(f,n), and otherwise, q_(s,n) may be interpolated as a weighted average between q_(f,n) (e.g., weighted by φ_(max)/φ) and q_(ω,n) (e.g., weighted by 1−φ_(max)/φ). The accurate interpolation for quaternion is a spherical linear interpolation, but the following linear approximation may be used instead:

q _(s,n) =q _(f,n) w _(f) +q _(ω,n)(1−w _(f))  (11)

where w_(f)=min(1,φ_(max)/φ).

The angle difference limit φ_(max) may be a monotonic function of the measured angular velocity's magnitude |ω_(n)|.

A dedicated apparatus/system may be used to implement the above-described methods. This dedicated apparatus/system may be part of a gaming system or an information system controlled by orientation of a device. For example, FIG. 9 illustrates a block diagram of such a system 900. System 900 includes sensors 910 (e.g., an accelerometer, a gyroscope and a magnetometer) mounted on the device and configured to acquire information leading to a measured angular velocity ω_(n) and an estimate of a current orientation q_(f,n) of the device. System 900 further includes data processing unit 920 configured to determine (A) an adjusted angular velocity Ω_(s,n) based on measured angular velocity ω_(n) and estimated angular velocity Ω_(f,n) to satisfy one or more predefined constraints, and (B) an adjusted estimate of current orientation q_(s,n) of the device obtained using adjusted angular velocity Ω_(s,n). The adjusted estimate of current orientation is mapped on variable x that may be used by a hardware component distinct from the device. Data processing unit 920 may be located on the device or remote, for example, on the hardware component (e.g., a display) using the variable x.

If system 900 is a gaming system configured to display an image to a user according to an orientation of a device (e.g., a handheld device), then data processing unit 920 is further configured to determine the image to be displayed according to the adjusted estimate of current orientation q_(s,n).

If system 900 is an information system controlled by orientation of a device, then data processing unit 920 is further configured to determine the position of a cursor on a screen based on the current orientation q_(s,n) of the device.

System 900 may also include a memory 930 storing a computer program product which, when executed by a data processing unit, executes any of the above-described methods. The memory may be any suitable computer-readable medium, including hard disks, CD-ROMs, digital versatile disc (DVD), optical storage devices, or magnetic storage devices such as a floppy disk or magnetic tape. Other non-limiting examples of computer-readable media include flash-type memories or other known memories. Accordingly, the exemplary embodiments may take the form of an entirely hardware embodiment or an embodiment combining hardware and software aspects.

The disclosed exemplary embodiments provide methods and apparatuses for determining an adjusted and (optionally) stabilized orientation of a device in a non-inertial reference system using measurements of motion sensors (such as 3D accelerometers and gyroscopes) and a magnetometer. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention. Other sensors could be used to measure orientations including angular position sensors, tilt sensors, cameras and so on. The various embodiments can be used when those alternate sensors are used. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the inventions. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. Orientation and angular velocity are exemplary features here, but similar methods to smooth a variable in a way that minimizes distress to either humans or other equipment or algorithms downstream. For example, similar methods could apply to translational movement's position and velocity. The variable does not have to be angular position or velocity nor does the targeted consumer have to be a human being.

Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 

What is claimed is:
 1. A method for smoothing evolution of a variable depending on an orientation of a device, the method comprising: determining an adjusted angular velocity (Ω_(s,n)) based on a measured angular velocity (ω_(n)) and an estimated angular velocity (Ω_(f,n)), such that to satisfy one or more predefined constraints; and determining a current value (x) of the variable according to an adjusted estimate of the current orientation (q_(s,n)) of the device obtained using the adjusted angular velocity (Ω_(s,n)).
 2. The method of claim 1, wherein the variable is a position of a cursor on a screen.
 3. The method of claim 1, wherein the variable defines an image displayed to a video-game user.
 4. The method of claim 1, wherein the adjusted estimate of the current orientation of the device, and the measured, estimated and adjusted angular velocities are expressed in a reference system defined relative to gravity direction and Earth magnetic field direction.
 5. The method of claim 1, further comprising: calculating the estimated angular velocity (Ω_(f,n)) using an estimate of a current orientation (q_(f,n)) of the device and an adjusted estimate of a previous orientation (q_(s,n-1)) of the device.
 6. The method of claim 5, wherein the estimate of the current orientation (q_(f,n)) of the device is obtained using a sensor fusion method.
 7. The method of claim 6, wherein the sensor fusion method uses measurements of a magnetometer and an accelerometer mounted on the device, and the measured angular velocity is obtained using a gyroscope mounted on the device.
 8. The method of claim 6, wherein the estimate of the current orientation (q_(f,n)) and the adjusted estimate of a previous orientation (q_(s,n-1)) are expressed as quaternions.
 9. The method of claim 1, wherein the one of more predetermined constraints include the adjusted angular velocity (Ω_(s,n)) to be substantially equal to zero when the measured angular velocity (ω_(n)) is substantially equal to zero.
 10. The method of claim 1, wherein the determining of the adjusted angular velocity (Ω_(s,n)) includes projecting the estimated angular velocity (Ω_(f,n)) along a direction of the measured angular velocity (ω_(n)), calculating a scaling factor (α) as a ratio of the result of the projecting and a magnitude of the measured angular velocity (ω_(n)); and if the scaling factor exceeds a predetermined first value (MaxScaling) or is below a predetermined second value (MinScaling), setting the scaling factor (α) equal to the predetermined first value or to the predetermined second value, respectively, wherein a magnitude of the adjusted angular velocity (Ω_(s,n)) is equal to the magnitude of the measured angular velocity (ω_(n)) multiplied with the scaling factor.
 11. The method of claim 1, wherein the determining of the adjusted angular velocity (Ω_(s,n)) includes: determining an auxiliary unit vector (v₂) based on a first unit vector (v₀) along the measured angular velocity (ω_(n)), a second unit vector (v₁) along the estimated angular velocity (Ω_(f,n)) and an angle (θ) there-between; and determining a unit vector (v) along the adjusted angular velocity (Ω_(s,n)) using the first, second and auxiliary unit vectors.
 12. The method of claim 11, wherein the unit vector (v) is the second unit vector (v₁) if the angle (θ) between the measured angular velocity (ω_(n)) and the estimated angular velocity (Ω_(f,n)) is less than or equal to a maximum angle (θ_(max)).
 13. The method of claim 12, wherein if the angle (θ) is larger than the maximum angle (θ_(max)), then the unit vector (v) is at the maximum angle from the measured angular velocity (ω_(n)), between and in a same plane as the measured angular velocity (ω_(n)) and the estimated angular velocity (Ω_(f,n)).
 14. The method of claim 13, wherein the determining of the adjusted angular velocity (Ω_(s,n)) includes projecting the estimated angular velocity (Ω_(f,n)) along a direction of the measured angular velocity (ω_(n)), calculating a scaling factor (α) as a ratio of the result of the projecting and a magnitude of the measured angular velocity (ω_(n)); and if the scaling factor (α) exceeds a predetermined first value (MaxScaling) or is below a predetermined second value (MinScaling), setting the scaling factor (α) equal to the predetermined first value or to the predetermined second value, respectively, wherein a magnitude of the adjusted angular velocity (Ω_(s,n)) is equal to the magnitude of the measured angular velocity (ω_(n)) multiplied with the scaling factor (α).
 15. The method of claim 14, further comprising: adjusting at least one of the predetermined first value (MaxScaling), the predetermined second value (MinScaling) and the maximum angle (θ_(max)).
 16. The method of claim 1, further comprising: outputting a stabilized current orientation which is equal to a stabilized previous orientation (q′_(s,n-1)) if a difference between the adjusted estimate of the current orientation (q_(s,n)) and the stabilized previous orientation (q′_(s,n-1)) is less than a predetermined threshold (m).
 17. The method of claim 16, wherein if a difference between the adjusted estimate of the current orientation (q_(s,n)) and the stabilized previous orientation (q′_(s,n-1)) exceeds the predetermined threshold (m), then the stabilized current orientation (q′_(s,n)) is equal to the adjusted estimate of the current orientation (q_(s,n)) after being shifted with the predetermined threshold, towards the stabilized previous orientation (q′_(s,n-1)).
 18. The method of claim 1, wherein the method is performed by a processor located in the device.
 19. The method of claim 1, wherein the adjusted estimate of the current orientation (q_(s,n)) of the device is obtained by adding to a previous orientation (q_(s,n-1)) of the device, a change thereof obtained by integrating the adjusted angular velocity (Ω_(s,n)) over a time interval.
 20. A system configured to display an image to a user according to an orientation of a device, the system comprising: sensors mounted on the device and configured to acquire information leading to a measured angular velocity (ω_(n)) and an estimate of a current orientation (q_(f,n)) of the device; a data processing unit configured to determine: an adjusted angular velocity (Ω_(s,n)) based on the measured angular velocity (ω_(n)) and the estimated angular velocity (Ω_(f,n)), such that to satisfy one or more predefined constraints, an adjusted estimate of the current orientation (q_(s,n)) of the device obtained using the adjusted angular velocity (Ω_(s,n)), and the image to be displayed to the user according to the adjusted estimate of the current orientation. 